            \begin{tikzpicture}
            %------------------------------------------------------------------------------------
            \centering
            \begin{scope}[sibling distance=25pt, level distance=25pt] 
            \Tree[.\node(a1){:=};
                 	[.\node(b1){b};]
                 	[.\node(b2){+};
                            [.\node(c1){a};]  
                            [.\node(c2){*};
                                [.\node(d1){2};]
                                [.\node(d2){5};]
                            ]
                        ]
                 ]
            \end{scope}
             % \node at ([xshift=-10em,yshift=0.5em]SYNBL.north){符号表};
            \node (a) at ([xshift=18em,yshift=-1em]a1.west){(1)(* \quad 2 \quad 5 \quad $t_1$)};
            \node (b) at ([xshift=0.1em,yshift=-2em]a.south) {(2)(+ \quad a \quad $t_1$ \quad $t_2$)};
            \node (c) at ([xshift=0.2em,yshift=-2em]b.south) {(3)(:= \quad a \quad $t_1$ \quad $t_2$)};
            \node (d) at ([xshift=-2em,yshift=3em]a1.west){b:=a+2*5};
            \node (e) at ([xshift=-1em,yshift=0em]d.west){如:};
            % \draw[blue!60, -{Triangle[width = 18pt, length = 8pt]}, line width = 10pt] ([xshift=2em,yshift=-5em]VALL.east) -- ([xshift=2em,yshift=5em]VALL.east);
            % \draw[yellow!60, -{Triangle[width = 18pt, length = 8pt]}, line width = 10pt] ([xshift=8em,yshift=-4em]a1.east) -- ([xshift=11em,yshift=-4em]a1.east);
            \node (e) at ([xshift=9.2em,yshift=-3em]a1.east){或};
            % \draw [->] (set a) to node [auto] {\textit{q}} (set b);
            
            % \begin{pgfonlayer}{background}
            % \node [fill=orange!10,fit=(a) (b) (c)] {};
            % \end{pgfonlayer}
            % \begin{pgfonlayer}{background}
            % \node [fill=yellow!10,fit=(a1) (b1) (b2)(c1)(c2)(d1)(d2)] {};
            % \end{pgfonlayer}
            % \begin{pgfonlayer}{background}
            % \node [fill=yellow!10,fit=(d)] {};
            % \end{pgfonlayer}
            
            %------------------------------------------------------------------------------------
            \end{tikzpicture}